1. Field of the Invention
The present invention relates to an image signal processing system and an electronic imaging device.
2. Description of the Related Art
The phenomenon as shown in FIG. 4 in which the light amount decreases in stages and substantially in concentric circles from the optical center of an image (the optical axis of the image-pickup optical system) toward the peripheral portion of the image surface is called shading (peripheral extinction). This is problematic in wide images in particular because the ability to delineate the image at the peripheral portion deteriorates.
In electronic imaging devices such as digital cameras, digital video cameras, and the like, by raising the gain of the output signal of an image pick-up element (CCD) gradually and in concentric circles from the optical center of the image toward the periphery, the peripheral extinction can be cancelled out, and the image can be corrected so as to become substantially the same brightness on the whole.
However, conventionally, because the gain of the peripheral portion is raised as compared with the central portion of the image surface, the gain of the noise at the peripheral portion of the image is also raised, and the S/N (signal-to-noise ratio) deteriorates. If the S/N at the peripheral portion of the image surface deteriorates, as a result, a new problems arises in that the image quality of the image on the whole is reduced.
In electronic imaging devices such as digital cameras and the like, usually, contour correction is carried out and the contour is enhanced, in order to show the image distinctly. However, with both of the above-described shading correction and contour correction, noise is generated in the actual image. Further, if strong shading correction is carried out, the amount of noise which is generated increases in proportion to the amount of correction.
There have been proposed shading correction devices in which noise at the peripheral portion of the image is made less conspicuous by, while carrying out the shading correction, suppressing the amount of contour correction at the peripheral portion of the image where strong shading correction is needed (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2001-339736 and 2001-167263).
However, the noise arising from shading correction and the noise arising from contour correction are intrinsically in different dimensions, and it is preferable that they respectively be processed appropriately. In particular, in the above-described method of suppressing the amount of contour correction in order to reduce noise at the peripheral portion of the image, the sharpness of the peripheral portion of the image decreases, which, as a result, leads to a reduction in image quality.